The present invention relates generally to power amplification, and more particularly relates to a variable output power supply for use, for example, in a highly efficient linear amplification system.
It is well-established that the linear amplification of signals having an amplitude-varying envelope is a notoriously inefficient process. This inefficiency arises, at least in part, from the fact that an amplifier employed for the amplification of an amplitude-varying signal must possess enough voltage headroom between positive and negative voltage supply rails of the amplifier to allow amplification of the peaks of the signal without clipping, which can result in distortion of the output signal of the amplifier.
In addition to distortion resulting from attempting to amplify a signal beyond the voltage supply rails of the amplifier, another known disadvantage of conventional power amplifiers is that when the amplified signal is at or near one or both voltage supply rails of the amplifier, the amplifier can become saturated and generally exhibit nonlinear operation. The nonlinear operation of a saturated amplifier causes an increase in intermodulation products, resulting in undesirable amounts of energy in frequency bands other than a frequency band intended for use. This undesirable energy is often referred to as Adjacent Channel Power (ACP). Since efficiency has traditionally been improved by driving an amplifier into saturation, it is evident that there exists a tradeoff between increasing amplifier efficiency and decreasing intermodulation products.
Typically, in order to amplify large amplitude signals, the positive and negative voltage supply rails of the amplifier are set to a predetermined level so as to provide sufficient headroom, such that an amplified output signal of the amplifier exhibits essentially no clipping and substantially no distortion. When the same amplifier is used to amplify small amplitude signals, however, the additional voltage headroom provided by the fixed voltage supply results in unnecessary power dissipation and inefficiency in the amplifier.
Accordingly, there exists a need for a linear power amplifier having improved efficiency, particularly in applications involving the amplification of signals having amplitude-varying envelopes. Furthermore, it would be desirable to achieve such efficiency while reducing intermodulation products, distortion and amplifier power dissipation.
The present invention provides techniques for increasing an efficiency of a linear power amplifier by dynamically varying an output voltage of a variable output power supply coupled to the power amplifier such that the variable output power supply maintains a minimum voltage headroom in the amplifier regardless of the amplitude of an input signal presented to the amplifier. In this manner, an efficiency and frequency response of the amplifier is advantageously improved and a power dissipation of the amplifier is significantly reduced.
In accordance with one aspect of the invention, a variable output power supply for use in a highly efficient linear amplification system includes an envelope detector having at least one input for receiving an input signal to be amplified. The envelope detector includes an output for generating a control signal that is representative of an envelope of the input signal. The variable output power supply further includes a controllable voltage source that is coupled to the envelope detector. The controllable voltage source generates an output voltage that is responsive to the control signal, such that the output voltage dynamically changes as a function of the envelope of the input signal. When employed in a linear amplification system, the variable output power supply provides a supply voltage to a linear amplifier which is dynamically adjusted in response to the envelope of the input signal so as to provide a substantially constant voltage headroom for the amplifier.
These and other features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.